The invention relates to a transparent or translucent thermoplastic moulding composition containing a dye mixture containing at least one dye of the structure 1a) and/or 1b) and at least one further dye selected from the group comprising perinone-based dyes differing from structure 1a) or 1b) and dyes based on a phthalocyanine, phthalocyanine complex or indanthrone.
The invention relates in particular to transparent or translucent thermoplastic moulding compositions containing such a dye mixture for the production of optical components, such as for example lenses, or optical fibres, which exhibit an elevated colour stability when exposed to LED light, as well as to their production and use.
The invention also relates to transparent or translucent moulding compositions containing such a dye mixture for the production of optical components, such as for example lenses, or optical fibres, wherein if a specific colour location is established by colour correction using the dyes according to the invention these moulding compositions demonstrate a higher transmission for LED light than moulding compositions corrected with other dyes.
Unlike conventional illuminants such as incandescent light bulbs or fluorescent tubes, LEDS have a different emission characteristic. Lenses, light guides or optical fibres often have to be used for applications which require the light beam to be directed. Alternatively or additionally, illuminants with LEDs as the light source generally contain a transparent or translucent housing section serving to cover the light source, which protects the light source and shields it from external influences such as dirt and dust.
Owing to their long life, low energy consumption and good light yield, LEDs are increasingly being used as illumination sources, for example in the automotive industry, aviation, interior lighting, facade design, etc.
Incandescent light bulbs are disadvantageous because of their poor efficiency in terms of light emission and high evolution of heat as well as their short life. Energy-saving bulbs are much more energy efficient, but because of their content of heavy metals, in particular mercury, they are highly environmentally damaging and have to be disposed of as special waste. Alternative concepts to conventional illumination sources and modules such as incandescent light bulbs or energy-saving bulbs are sought after in terms of sustainability and energy efficiency.
Semiconductor technology (in the form of LEDs, OLEDs or electroluminescent films) offers an alternative illumination source which does not exhibit these disadvantages and which in addition has a long life and high energy efficiency. LEDs are a preferred use of semiconductor technology as a light source.
LEDs radiate light at a wavelength that is dependent on the semiconductor material and doping, such that almost monochromatic light, even in the infrared or UV range, can be generated with LEDs.
To generate visible white light, which is known to be a mixture of different wavelengths, the monochromatic light of LEDs therefore has to be “converted” (for example by additive colour mixing), which in principle is possible by various means:                1. Colour mixing by combining a blue, a red and a green emitting LED to form RGB (red green blue) modules, the combined perceived light impression of which is white.        2. By means of luminescence techniques in which all or part of the LED radiation is converted to other wavelengths using phosphors for example.        
Thus white light can be generated from an LED which radiates blue light in the visible range by the addition of a single phosphor, which converts part of the radiation in the blue range into red/yellow light. This form of generating white light is preferred for commercial applications for cost reasons and because of the high efficiency of blue LEDs.
Alternatively, white light can be produced from UV light generated with LEDs with the aid of three different phosphors emitting wavelengths corresponding to an RGB module. If this method is used, compositions are preferred which also have elevated stability in respect of UV radiation, in other words which are UV-stabilised for example.
Where necessary the above light sources can be still further modified to establish an overall colour impression other than “white” in LED modules. This modification can take place for example by:                Combination with a phosphorescent dye or        Combination with additional light sources with a different emission characteristic.        
If transparent or translucent plastics are used for lenses, optical fibres, covers or other components of a lighting device, the focus is on the light stability of the light source used. This is preferably in the range that is visible to the human eye, since these lighting devices are designed for use by people. Emissions outside the visible range mean a loss of energy and hence a reduced efficiency of the light source. There is therefore a need for a transparent or translucent plastic composition having high colour stability in respect of the emission spectrum of such a light source. In the case of a thermoplastic composition the production of such components also requires good flowability in order to be able to manufacture complex geometries by simple means. Such components should be able to be connected to the other elements of the lighting device by means of integrated elements such as plug-in or screw connectors, for example, and therefore require good mechanical strength. In addition, elevated heat resistance is required in order to be able to withstand the usage temperatures of the lighting device on a lasting basis without changes to the optical, geometric or other properties.
Lenses, optical fibres, transparent or translucent covers and other transparent or translucent components in illuminants can be produced from transparent or translucent polymers. Injection mouldable transparent or translucent thermoplastics and thermoplastic compositions selected from the group encompassing polycarbonate (PC), copolycarbonate, polyester carbonate, polystyrene (PS), styrene copolymers, polyalkylenes such as polyethylene (PE) and polypropylene (PP), aromatic polyesters such as polyethylene terephthalate (PET) and polybutylene terephthalate (PBT), PET-cyclohexane dimethanol copolymer (PETG), polyethylene naphthalate (PEN), poly- or copolymethyl methacrylates such as polymethyl methacrylate (PMMA), polyimides (e.g. PMMI), polyether sulfones, thermoplastic polyurethanes, cyclic olefin polymers or copolymers (COP or COC), or mixtures of the cited components, provided these mixtures are transparent or translucent, are suitable in particular for this purpose.
Additional dyes are frequently added to such transparent or translucent materials in order to change the colour impression or colour temperature of the light.
Stable dyes are known per se and are described in the literature. Thus U.S. Pat. No. 6,476,158 describes opaque, i.e. not transparent or translucent, polycarbonate-based compositions (polycarbonate-polyester blends) having a particularly high stability with regard to artificial weathering. Dyes which have a high weathering resistance or which contribute to maintaining gloss after weathering are also mentioned in this application.
U.S. Pat. No. 6,355,723 discloses similar dyes to U.S. Pat. No. 6,476,158, which because of their thermal stability are suitable in principle for being incorporated into polycarbonate. However, no information is given about stability with regard to radiation of any type and in particular in the visible range.
In the production and use of optical components such as lenses, optical fibres and covers made from compositions known in the prior art, however, it was found that they are changed by LED light. In particular, the optical properties, e.g. the yellowness index (YI), are changed to an unacceptable extent.
This is particularly unexpected, since LED light contains no significant proportion of UV radiation (<360 nm), which along with thermal influences is substantially responsible for the discoloration of transparent or translucent thermoplastics.
Surprisingly it was found furthermore that the anthraquinone-based dyes, which are described in U.S. Pat. No. 6,476,158 as being lightfast and colour-stable in polycarbonate, are inadequately stable in respect of light in the visible range, in particular LED light. The person skilled in the art would assume that dyes which in particular are also stable in respect of sunlight, which contains a high-energy and potentially highly damaging UV component (<360 nm), should really also be suitable for such applications exposed to LED light.
This poses the unexpected problem for the person skilled in the art that other dye classes or substances too, such as methine dyes or phthalocyanines, which are generally believed to exhibit high lightfastness and colour stability in respect of weathering influences, are not necessarily suitable for applications involving exposure to LED light.
Furthermore it is often necessary to combine a plurality of dyes to establish a certain colour impression.
Thus it is not obvious to the person skilled in the art from the existing prior art which dyes are stable in respect of LED radiation. Therefore no dyes or dye combinations for use in components exposed to LED light are known from the prior art.